1. Field of the invention
This invention relates to a radiation detector and a process for its production. More particularly, it relates to a radiation detector having at least i) a wavelength converter as typified by a phosphor (fluorescent substance) that converts radiation such as .alpha.-rays, .beta.-rays, .gamma.-rays and X-rays into light of a desired wavelength range, such as visible light; and ii) a photoelectric transducer having a sensitivity to the light thus converted, and also relates to a process for its production.
2. Related Background Art
Radiation detectors have been utilized in a variety of fields. Radiation detectors that utilize X-rays among various radiations are widely applied and developed in medical instruments, chemical analyzers and nondestructive inspection instruments. Typical apparatus having been put into practical use include, e.g., chest and stomach X-ray diagnostic apparatus, X-ray tomographs, circulatory organ X-ray diagnostic apparatus, X-ray fluoroscopes, X-ray diffraction apparatus, X-ray nondestructive inspection apparatus, and X-ray residual-stress analyzers.
As detecting elements in these apparatus, it is common to use, e.g., those employing X-ray films, those employing gas detectors, those comprised of combination of a phosphor with a photomultiplier, and those comprised of combination of an image intensifier with a film camera or television camera.
In recent years, in order to make apparatus compact and to make them digital to permit combination with image processing, developing detectors comprised of a combination of a phosphor with a solid photoelectric transducer is desired. Detectors are highlighted which employ imaging plates as substitutes for films, or which employ CCD solid image pickup devices as substitutes for analog television cameras. In these devices, the image information detected can be handled as digital signals in accordance with electric signals that carry the information. Hence, image data can be analyzed or processed with ease, thus the radiation detectors have a high performance.
However, no animation can be made when the imaging plates are used. When the CCD camera system is employed, imaging intensifiers are so bulky that the apparatus cannot be easily made compact. Especially in medical instruments, radiation detectors adaptable to large areas are needed. For example, in chest X-ray fluoroscopes, a large area of at least 46 cm square must be ensured. Accordingly, developing a radiation detector that can detect radiations over a large area, and at the same time can make the apparatus itself compact and is adaptable to animation is desired. For example, use of an amorphous silicon hydride highly sensitive photoelectric transducer as the photoelectric transducer makes the detector adaptable to large areas with ease. Moreover, combination of such a device with a phosphor enables one-to-one direct detection of large-area radiations with high sensitivity without providing any optical reduction system or image intensifier between them. Hence, it is possible to obtain a large-area, compact and animation-adaptable radiation detector. For example, one may contemplate combining a two-dimensional photoelectric transducer as disclosed in European Patent Publication No. 0660421, with a phosphor.
In the large-area radiation detector as described above, a phosphor sheet coated with a phosphor is bonded to a substrate having a photoelectric transducer, the former being bonded to the latter over its whole area by the use of an adhesive. FIG. 1 diagrammatically illustrates a cross-section of an example of such a radiation detector. In FIG. 1, reference numeral 1101 denotes a glass substrate; 1102, a layer provided with photoelectric transducers and so forth; 1103, an impurity preventive layer formed of polyimide or the like; 1104, an epoxy type adhesive; 1105, a phosphor protective layer; 1106, a phosphor; and 1107 a base sheet.
Now, some problems may occur when such a radiation detector is made large-area or the phosphor and the photoelectric transducer are integrated. More specifically, a problem may arise such that the difference in coefficient of thermal expansion between the glass substrate and the phosphor sheet or adhesive becomes significant to cause cracks in the phosphor sheet or deflection of the substrate.
Another problem may occur such that it is difficult to form a uniform adhesive layer over the whole substrate surface and it is difficult to keep uniform the flatness of the phosphor sheet, so that impurities in the adhesive may adversely affect the phosphor or the underlying devices. Hence, it may be required to further add a protective film. Namely, this results in addition of a component between the phosphor sheet and the substrate.
The support member of a base sheet used has a thickness of about 1 mm in order to ensure an appropriate strength and a uniform flatness. In usual instances, the base sheet is formed using an aluminum plate having a high X-ray transmittance, which also serve as a plate for reflecting light produced in the phosphor. Hence, the whole radiation detector tends to be heavy-weight.
Another problem may occur such that a useless gap is formed between the photoelectric transducer and the phosphor because of the protective film of the phosphor sheet, adhesive layer and so forth, resulting in a decrease in utilization of light and causing blurred images because of scattering of light that occurs there. Especially in the case of medical instruments, it is required to limit radiation doses to human bodies. Also, the conversion efficiency of the phosphor, as conversion of radiant energy into light, is about 10%. Thus, under conditions that the light should be effectively utilized as far as possible, such a decrease in efficiency may become problematic.
In addition, the phosphor has a moisture absorption and most phosphors deteriorate upon absorption of water, thus the phosphor may adversely affect devices concurrently because of the moisture absorption of the adhesive.
More specifically, as stated above, as the area of the radiation detector is increased or the phosphor and the photoelectric transducer are integrated, it has become necessary to take the following points into account.
(1) Cracking of the phosphor sheet and deflection of the substrate, caused by the difference in coefficient of thermal expansion between the glass substrate and the phosphor sheet or adhesive; PA0 (2) The formation of a uniform adhesive layer over the whole substrate surface, and the flatness of the phosphor sheet; PA0 (3) The effect of impurities in the adhesive upon the phosphor and underlying devices; PA0 (4) The component between the phosphor sheet and the substrate; PA0 (5) The weight of the whole radiation detector; PA0 (6) The gap formed between the photoelectric transducer and the phosphor because of the protective film of the phosphor sheet, adhesive layer and so forth, the utilization of light, and the blurred images caused by scattering of light; and PA0 (7) The moisture absorption of the phosphor, the deterioration of the phosphor upon absorption of water, and the moisture absorption of the adhesive.